This invention relates to an apparatus with which, by means of a drawing operation, as end portion is formed on a metal tube of given diameter which has a diameter less than said given diameter.
In carrying out normal drawing operations on a metal tube in order to obtain therefrom a tube of smaller diameter, it must be provided with an end portion having a diameter substantially less than that of said tube, the purpose of which is to enable the tube to be easily inserted into the dies in the actual drawing apparatus, and to cooperate with parts of suitable means for feeding the tube through the die.
Said tubes are prepared for such drawing operations by a pointing machine, with which the end of each tube is given an actual point which comprises a substantially cylindrical end portion connected to the tube by another substantially conical portion.
Said apparatus comprises a die through which the end of said tube is passed and which has a minimum diameter less than the tube diameter and substantially equal to the diameter of the end portion of the point to be obtained.
In such apparatus, the tube is usually held fixed relative to the apparatus frame by means of suitable jaws operated by a first hydraulic cylinder while the die is moved axially relative to the tube. This die is usually carried by a slide mobile on suitable columns in the apparatus by the action of a second hydraulic cylinder. Apparatus of the aforesaid type present certain drawbacks.
Firstly, the force necessary in order to form said point on the end portion of the tube by means of permanently deforming the tube material is very large because of the very high radial pressures which the die surfaces have to apply to the tube surface under formation in order to reduce its relative diameter to the extent necessary for forming said point. These pressures are high both because of the large ratio of the initial tube diameter to the final point diameter, and because of the considerable mechanical strength of the material, due to the fact that said tubes are constructed of unannealed metal, and thus have a very high hardness and ultimate tensile stress.
Because considerable forces are necessary for carrying out the described forming operation, the relative apparatus must be of large overall dimensions. In this respect, the closure force of said jaws which retain the tube during the operation is proportional to the drawing force, as the connection between the tube and jaws is provided only by the friction force generated between the contacting surfaces thereof. Because of the high drawing force and low coefficient of friction (in this case of the order of 0.10), the closure force of the jaws must be very large, with the consequence that these jaws and the relative hydraulic cylinder must be of considerable size. The die support slide and its operating cylinder, which must be able to generate said drawing force, must also obviously be very large.
Finally, the point obtained by the described apparatus is not always completely uniform because of undesirable movements of the material of said point, and sometimes it does not have sufficient mechanical strength to enable it to be used for the purposes for which said point is intended.